This application is related to Japanese Patent Application No. 2000-153572 filed in May 24, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a solar cell and a method for fabricating the same; more specifically, it relates to a solar cell having a bypass function, which protects the solar cell from a reverse voltage applied when the solar cell is light-shielded in the solar cell module.
2. Related Art
In order to construct a photovoltaic power generation system in which a high current and high voltage is required, a conventional solar cell module has been provided which includes a plurality of serial or parallel connected solar cells to obtain the desired output current and output voltage. However, in case a solar cell in such a solar cell module is light-shielded by a shadow of something, a reverse voltage (reverse bias) generated in the other cells is applied to the light-shielded solar cell.
If the reverse voltage should exceed a reverse breakdown voltage of the cell, a breakdown occurs on the junction in the cell as to cause a large current flow, thereby resulting in a drop in power output of the entire solar cell module.
In case of solar cell modules for use in space, for instance, during the attitude control of a satellite, a part of the satellite or a structure such as an antenna, etc., may cast a shadow on the solar cell module. In case of terrestrial use, the shadow occurs due to, for instance, the neighboring buildings or the adhesion of the droppings of fowls. In order to prevent the breakdown of the solar cells due to the applied reverse voltage, there are known solar cell modules having bypass diodes externally attached to each solar cell or to each group of solar cells. Furthermore, there is also known a so-called diode integrated solar cell in which bypass diodes are integrated to the solar cells (see, for instance, Japanese Unexamined Patent Publication No. Hei 4(1992)-179169 or Japanese Unexamined Patent Publication No. Hei 5(1993)-110121).
However, in case the bypass diodes are attached externally to the solar cells, there occurred a problem of lowering the mount density of the solar cells on a substrate due to an increase in the number of mounted components, or another problem of increasing the fabrication steps of the module.
On the other hand, in case the diodes are integrated to the solar cells, the fabrication steps become so complicated because the bypass diodes are integrated together with the solar cells on the same silicon substrate, and this leads to a problem of increasing the fabrication cost.
Furthermore, the bypass diodes of the conventional type have a Zener diode structure. This structure is characterized in that the PN junction that is formed by two high density impurity diffusion layers has an acute impurity distribution. This leads to a problem of causing fluctuation in current characteristics due to a fluctuation in one or both of the diffusion layers.
In the light of the aforementioned circumstances, the present invention provides a solar cell comprising: a semiconductor substrate of a first conductive type; a first region of a second conductive type provided on a surface of the substrate; and a bypass function region including a second region of a second conductive type provided on the surface of the substrate and spaced a predetermined distance from the first region, and a third region for setting the substrate and the second region at the same potential; wherein when a reverse voltage is applied between the substrate and the first region, the bypass function region forms a bypass circuit between the substrate and the first region using a depletion layer formed inside the substrate by the reverse voltage.